The present invention relates to the field of photonics, and more particularly to low-dimensionality semiconductor laser devices capable of emitting different frequencies.
Presently, very efficient and compact laser sources can be obtained using semiconductor laser diodes based on 2-dimensional quantum well(s) in their active gain region. Such state-of-the-art semiconductor laser diodes can produce hundreds of milliwatts of laser light emitted over a narrow range of wavelengths of a few nanometers (nm) or smaller. Typically, to obtain a different wavelength, a distinct laser diode must be fabricated with the appropriate quantum well(s) in its active region. For several applications, a wide range of wavelengths are necessary. This limits the usefulness of semiconductor laser diodes based on quantum wells because the 2-dimensional density-of-states of the electronic structure results in a gain spectrum which can be tuned at most by tens of nanometers using external cavities, or using integrated tuning elements.
The current state-of-the-art technology used to obtain laser sources tunable over hundreds of nanometers using external cavity configurations with a solid-state crystal such as a Ti-Sapphire lasers (Ti-Saph lasers), or with dyes mixed in a liquid medium (Dye lasers). These lasers have major limitations because they are not compact and are very inefficient since they have to be aligned and optically pumped with another powerful laser operated at shorter wavelengths.
There exists a real need for compact and efficient lasers, tunable over a broad range of wavelengths for multimedia and telecommunication applications, as well as for diagnostic and research/development tools. New applications will also emerge with the development and availability of such laser sources.
It is therefore an object of the invention to provide an apparatus and method capable of generating laser light tunable over a wide range of wavelengths in a compact and efficient way.
Unlike the density-of-state of bulk material and of quantum wells, the electronic configuration of low-dimensional nano-structures, herein defined as quantum wires (one- or quasi-one dimensional structures) or quantum dots (zero or quasi-zero dimensional structures), will allow the saturation of their reduced density-of-states over a wide range of energies because the total number of available states is orders of magnitude smaller than for quantum wells. This will permit the production of population inversions and lasing over wide range of wavelengths. Also, it is possible to produce Self-Assembled Quantum Dots (QD) by epitaxy using highly strained semiconductors, and to have good 1o control over their zero-dimensional density-of-state. Such quantum dots can be grown in a laser diode configuration with conventional techniques, and the carriers will be injected electrically in the QD laser diode. To obtain the tunability in such a QD laser diode having a wide gain spectrum, an external cavity is used. The resulting QD tunable external cavity (QD-TEC) laser retains the efficiency and convenience of conventional semiconductor laser diodes, and yet is tunable over hundreds of nanometers by choosing the low-dimensional electronic structure of the QD and the optical properties of the external cavity.
Accordingly in a broad aspect the invention provides a laser system comprising a laser diode with low dimensional quantum structures for emitting light over a wide range of wavelengths, a wavelength-selective element for selecting a wavelength of interest emitted by said laser diode, and an external cavity resonant at a wavelength selected by said wavelength-selective element so that the system generates laser light at said selected wavelength.
The wavelength-selective element used to tune the laser output may consist of an a diffraction grating, a prism, a birefringent element, an etalon, or a dispersive element.
One dimensional or quasi-one-dimensional structures can be obtained from coupled zero- or quasi-zero dimensional structures, or from other techniques which can produce quantum wires.
In operation the application of an electric field causes charged-carriers to be injected from contact layers into an active region of a semiconductor heterostructure containing quantum dots or quantum wires. Then photons originating from the radiative recombination of the charged carriers in the active region are emitted. The photons are confined in the cavity designed with tunable wavelength-selective elements which are adjusted to support a lasing output over the selected wavelengths.
The laser diode and the wavelength-selective element are preferably located within the external cavity in such a way that the laser light is emitted from the laser diode passes through the wavelength-selective element and resonates within the external cavity by passing one or several times through the laser diode and the wavelength-selective element, to finally exit out of the external cavity through one or several outputs. In a preferred embodiment the laser diode is a quantum dot (QD) laser diode.
The external cavity may be formed either in part from a facet of the laser diode, and/or in part from the said wavelength-selective element as an output-coupler, and/or from specially designed optical components as high reflectors, and/or folding mirrors, and/or output couplers.
The QD laser diode preferably comprises multiple layers of semiconductor materials including a least one quantum dot layer in an active region between an electron emitter layer, allowing the injection of electrons towards the quantum dots, and a hole emitter layer, allowing the injection of holes towards the quantum dots. The composition and doping of the materials is chosen so that the relative optical constants, bandgaps, and conductivity of the layers establish an effective guiding of the optical modes in a cavity formed perpendicular to the plane of the layers, as well as efficient carrier injection when an electric field is applied with the proper forward-bias polarity.
In the case where multiple quantum dot layers are used in the active region, barriers separate the quantum dot layers. The electron and hole emitter layers are preferably doped n-type and p-type respectively to act as a reservoir of charged carriers and to conduct the current necessary for the operation under bias. The electron and/or hole emitter layers can be composed of several layers or regions to vary the composition and/or doping, to optimize the optical and electrical properties of the QD laser diode.
The active region is preferably not doped to minimize loses of the guided optical modes. Intermediate layers with chosen bandgap and doping can also be introduced between the active region and the emitter layers to tailor the optical guiding and the optical and electrical properties of the laser diode. The current injection and the optical mode guided in the QD laser diode material are preferably confined laterally to tailor the electrical, thermal, and optical characteristic of the QD-TEC laser. The current injection in the QD laser diode material might preferably be confined longitudinally to tailor the electrical, thermal, and optical characteristic of the QD-TEC laser. The longitudinal confinement of the optical mode guided in the QD laser diode material is preferably adjusted by changing the reflectivity of a front and a back facet individually to tailor the electrical, thermal, and optical characteristic of the QD-TEC laser. It might be preferable to regulate the temperature and/or remove excess heat generated by the operation of the QD laser diode with the help of a temperature regulating device.
The wavelength-selective element is preferably designed to be adjustable to a bandpass over the gain spectrum of the QD-TEC laser. For the wavelengths selected in the bandpass, lasing will be achieved from a net optical gain which will be obtained before the photon escape the cavity, whereas the wavelengths outside the bandpass will not lase because the attenuation will be larger than the gain. The selected bandpass can be changed by simply adjusting the wavelength-selective element to obtain lasing at the various wavelengths available from the gain spectrum of the QD-TEC laser. In some embodiments, it might be preferable to build the wavelength-selective tuning element integrated to the QD laser diode.
The external cavity preferably provides the appropriate optical feedback to yield tunable lasing over a large portion of the gain spectrum of the QD laser diode. It may comprise at lease one back mirror and one output coupler but might use more complex configurations with several folding mirrors and/or output couplers to provide the desired optical characteristic and mode profiling functions. In some embodiments, it might be preferable to integrate part of the external cavity into the QD laser diode and/or to the wavelength selective tuning element.
Also, the size/shape of the quantum dots and the number of quantum dots per unit area are adjusted from the growth parameters in conjunction with the choice of quantum dot material, of barrier materials, of the number of quantum dot layers, of external cavity parameters, and of wavelength-selective element to achieve the desired tuning range and while optimizing the lasing efficiency for the wavelengths of interest. Similarly, the choice of the barrier material, the doping profiles, potential height, and barrier thickness is adjusted in conjunction with the quantum dot size to set the gain spectrum of the QD laser diode, to select a balance the laser efficiency and modulation speed, and to achieve the desired growth mode in the self-assembling growth. For multiple layers of quantum dots very thin barriers result in coupled zero-dimensional states in vertically self-organized quantum dots; thicker barriers result in isolated zero-dimensional states in vertically self-organized quantum dots, and thick barriers will result in isolated zero-dimensional states in uncorrelated independent quantum dot layers.
In another aspect the invention provides a method of producing low-dimensionality laser diodes having an adjustable gain spectrum based on a quantum material with low-dimensional density-of-states which relies on self-assembled quantum dots obtained by spontaneous island formation during epitaxy of highly strained semiconductors, comprising selecting a barrier material and a quantum material such that the degree of lattice-mismatch dictates a critical thickness required to obtain spontaneous island formation, and the bandgap difference determines a possible number of confined states in conjunction with the energy spectrum of the low-dimensional states; growing some thickness of said barrier material in an active region between an electron emitting layer and a hole emitting layer on a substrate, said electron and hole emitting layers having a lattice constant close to that of said substrate; depositing, at a specified growth rate, said quantum material at a temperature which will produce quantum dots having the appropriate size and shape to obtain said low-dimensionality density-of-states; ceasing the growth of said quantum material after the desired number of quantum dots per unit area is reached; waiting a specified amount of time to allow for the self-assembling growth to form the quantum dots in shapes and sizes which will give said low-dimensionality density-of-states; and growing some thickness of said barrier material to cover the quantum dots and return to a planar growth front at a substrate temperature which may be varied during the growth and which will optimize the quality of the quantum dots.
In a preferred form of the invention the layers of the semiconductor materials are grown on a substrate from materials consisting essentially of gallium, indium, aluminum, arsenic, phosphorous, and possibly nitrogen, using known techniques such as molecular beam epitaxy, or metalorganic chemical vapor deposition, or chemical beam epitaxy, with dopant such as silicon, beryllium, or others. On GaAs substrates, the quantum dot material can be InGaAs, AlInAs, InP, or other alloys of AlGaInAsP, with barriers of AlGaAs or AlGaInP. On InP substrate, the quantum dot material can be InGaAs. Alloys with nitrogen can be used with the above III-V in cases where different band gap materials are desirable. On group IV substrates, Si can be used for the barrier, with the III-V alloys mentioned above for the quantum dot material. The substrate is needed to give structural integrity to the very thin layers of the QD laser diode and to allow proper crystal growth.
The QD-TEC laser is preferably powered by electrical energy, and lasing at various wavelengths is obtained by adjusting the tuning element with the help of some mechanical components or some electro-optical actuating devices which can be calibrated and/or computerized. The optical characteristic of the output laser beam are preferably adjusted with the parameters of the external cavity. The zero-dimensional density-of-state which dictates the gain spectrum of the QD-TEC laser is controlled from the symmetry and shape of the self-assembled quantum dots which preferably takes a form resembling an hemispherical cap, a lens shape, a disk shape, a pyramidal or truncated and/or rounded pyramid shape. The self-assembled quantum dots are preferably obtained with the spontaneous island formation during the epitaxy of highly strained semiconductor materials. The details of the self-assembling growth are preferably arranged to establish the shape, symmetry, and size of the quantum dots and therefore set the quantum dot energy levels to optimize the lasing in the desired spectral range.
The low-dimensionality density-of-states can spans a wavelength range as small as 10 nm or as large as 500 nm. The specified range can be set at wavelengths between 0.4 micron to 2.0 micron, by using a GaAs substrate, an InP substrate, or another appropriate substrate, and by using Alxw(1-kv)Ga(1-xw)(1-xv)InxvAs(1-xu)Pxu semiconductor alloys and/or nitrogen containing alloys.